Epitaxially-grown GaN junction field effect transistors

Junction field effect transistors (JFETs) are fabricated on a GaN epitaxial structure grown by metal organic chemical vapor deposition (MOCVD). The dc and microwave characteristics of the device are presented. A junction breakdown voltage of 56 V is obtained corresponding to the theoretical limit of the breakdown field in GaN for the doping levels used. A maximum extrinsic transconductance (g_m ) of 48 mS/mm and a maximum source-drain current of 270 mA/mm are achieved on a 0.8 μm gate JFET device at V_GS=1 V and V_DS=15 V. The intrinsic transconductance, calculated from the measured g_m and the source series resistance, is 81 mS/mm. The f_T and f_max for these devices are 6 GHz and 12 GHz, respectively. These JFET´s exhibit a significant current reduction after a high drain bias is applied, which is attributed to a partially depleted channel caused by trapped hot-electrons in the semi-insulating GaN buffer layer. A theoretical model describing the current collapse is presented, and an estimate for the length of the trapped electron region is given